A new introduction.

After a couple of months of settling time, here's a new basic
introduction to Staapl.
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Staapl is a metaprogramming system based on the Forth and Scheme
programming languages. Staapl currently contains the following
distinct layers:
* A compiled Forth-like language for the Microchip PIC18
microcontroller architecture.
* A language for macro definition and instantiation (target code
generation) using Forth syntax. Used to implement the PIC18, and
written in terms of Coma.
* A stack-based macro language called Coma. It can be used for the
specification of Forth-like stack-based languages ("specification
by compiler"). Coma primitives generate and process sequential
imperative machine language (a real stack machine or a RISC
machine emulating a stack machine). Coma contains mechanism for
creating primitives that support partial evaluation based on
pattern matching. Coma's composition mechanism (concatenation)
can be manipulated from Scheme using hygienic quasiquotation.
Coma is written as an extension of Scat.
* An untyped stack language called Scat. Scat is written as an
extension of the Scheme programming language (PLT Scheme), and is
very similar to the Joy language.
These layers comprise the layered organization of the stack languages.
Most of the highlevel code is Scheme, and specialized macros written
in Scheme.
How does Staapl differ from its sources of inspiration?
* Staapl's idea of Forth differs from traditional Forth in that it
is heavily macro-based: all "words" in Staapl's Forth are macros,
and macro and procedure definitions are (mostly) the same:
procedures are instantiated macros.
* Scat is not Joy, because Scat's code is opaque, while Joy allows
for direct introspection and creation of code objects using list
operations.
* Coma is not just a macro assembler. Macros are _programs_ that
can operate on data (base machine language programs) in arbitrary
ways.
* Staapl is very different from metaprogramming tools based on
lexical scoping (i.e. MetaML). Staapl uses using a lexically
scoped metalanguagelanguage (Scheme) to metaprogram a stack
language which lacks local variables, avoiding problems with
lexical scope of the target language.